Human Responses to Different Built Hyperthermal Environments After Short-Term Heat Acclimation

Abstract

Hyperthermal environments are encountered in many situations, and significant heat stress can exacerbate the fatigue perception of individuals and potentially threaten their safety. Heat acclimation (HA) interventions have many benefits in preventing the risk of incidents. However, whether HA interventions in specific environments can cope with other different hyperthermal environments remains uncertain. In this study, forty-three young male participants were heat-acclimated over 10 days of training on a motorized treadmill in a fixed hyperthermal environment, and they were tested in different hyperthermal environments. Physiological indices (rectal temperature (T_r), heart rate (HR), skin temperature (T_sk), and total sweat loss (M_sl)) and subjective perception (rating of perceived exertion (RPE) and thermal sensation votes (TSVs)) were measured during both the heat stress test (HST) sessions and HA training sessions. The results show that HR and T_sk significantly differed between pre- and post-heat acclimation (p < 0.05 for all) following the acclimation program. However, after heat acclimation training, the reduction in T_r (ΔT_r) was more notable in lower-ET* environments, and M_sl showed distinct changes in different ET* environments. The RPE and TSV decreased after HA interventions, although the difference was not significant. The results indicate that HA can effectively reduce the peak of physiological parameters. However, when subjected to stronger heat stress, the improvement effects of heat acclimation on human responses will be affected. In addition, HA can alleviate physiological thermal strain, thereby reducing the adverse effects on mobility, but it has no effect on the supervisor’s ability to perceive the environment. This study suggests that additional HA training can reduce the risk of activities in high-temperature environments but exhibits different effects under different environmental conditions, indicating that hot acclimation suits have selective effects on the environment. This study provides recommendations for additional HA training before high-temperature activities.

1. Introduction

Hyperthermal environments are commonly encountered in many situations [1]. Non-air-conditioned environments induce intense and irreversible heat stress in humans who perform manual labor. Both outdoor and indoor production face the challenge of hyperthermal environments. Such environments cause serious damage to occupational health, and they increase the risk of accidents in the workplace [2]. Personnel in high-temperature environments are often prone to excessive fatigue due to the physical and mental strain caused by intense heat stress, which, in turn, increases the risk of workplace accidents [3]. Intense heat stress also has adverse effects on people’s perception and productivity [4]. Consequently, there is an increasing need to develop reasonable measures to protect human health and safety, and to this end, researchers are gradually focusing on the thermal/heat adaptability of humans in hyperthermal environments.

Some precautions should be taken to improve human heat adaptability in hot−humid environments. After a period of heat acclimation (repeated artificial heat exposure) or heat acclimatization (continuous heat exposure), changes in rectal temperature, heart rate, sweat osmotic pressure, and sweat loss are considered the main signs of complete physiological acclimation [5]. Therefore, heat acclimation (HA) is an effective way to improve fatigue perception [6], core temperature [7], and sweating rate [8], as well as heart rate [1]. There are two types of HA training according to the duration: long-term HA training and short-term HA training [9]. Previous studies have proposed various effective training programs, such as the “constant work rate regimen” [9], “self-regulated exercise regimen” [10], and “controlled-hyperthermia regimen” [11]. Different rates or intensities of training have different effects. As repeated thermal stimuli induce protective physiological responses in the human body, heat acclimation is a common method used to mitigate heat stress [12]. The acclimated state is commonly defined by the responses of the core temperature, heart rate, or sweating rate to continuous heat stimulation [13]. Hence, it is clear that HA phenomena occur in the human body, and it is important to reveal the regular pattern of HA [14].

To determine the effects of heat acclimation on human responses, many studies have focused on heat acclimation training [15,16,17]. As soon as physiological parameters exceed limit values, acute heat disease and injury will occur [18]. The occurrences of heat-related diseases in response to exercise−heat stress increase dramatically in hot environments [19]. In addition, previous studies [20,21,22,23] have indicated that heat acclimation can decrease human subjective perception when working in hot−humid environments, including significantly decreasing thermal and fatigue sensations. The passive heating of the body, such as via saunas [24] and hot water immersion [25], can provide an alternative and more accessible heat adaptation strategy. A sauna bath is traditionally regarded as a health-promoting environment; however, it has a disadvantage in that it is not a suitable place for exercise. However, the heat acclimatization cycle of these methods is relatively long, and it also cannot be maintained for a long time. Although heat acclimatization has been proven to limit human thermal strain and provide effective ergonomic assistance, most studies have focused on the effects of long-term heat acclimation, which requires more than 10 days of heat exposure [11].

The setting of HA training and environmental conditions involves a variety of factors; however, to date, there is no unified standard at home or abroad, and consensus has not been reached in a small number of military and sports fields. Chalmers et al. [26] aimed to explore the optimal training mode for competitive sports athletes by using a system evaluation and meta-analysis, but they only obtained a fuzzy mode, i.e., high-intensity exercise training for 60 min every day can effectively improve aerobic exercise performance. Many studies have demonstrated the benefits and effects of heat acclimation training; furthermore, many scholars have conducted research on the training mode [9,16,23]. However, the impact of environmental conditions has been ignored, which is the most important issue in this study. The importance of the training mode is undeniable, but if the impact of the environmental conditions on the training results is set aside, then the effects of training patterns are weakened.

In summary, extensive research has been conducted to explore the effects of heat acclimation. However, there is no research specifically focusing on environmental condition setting strategies for heat acclimation training. In particular, the impact of environmental parameters on the efficiency and effects of heat acclimation training has always been a relatively neglected research field. Moreover, it is still unknown whether heat acclimation based on the same environment can cope with the adverse effects of different high-temperature environments.

Within this framework, the primary aim of this study was to investigate the effects of different levels of heat stress on the ability to acclimatize to heat based on physiological and psychological parameters. These variables were analyzed before and after HA training in order to identify the correlation between heat acclimation and physiological and psychological effects and to explore the effects of HA and environmental conditions on these variables.

2. Materials and Methods

2.1. Climate Chamber

This study was conducted in a steady thermal indoor environment climate chamber. The chamber was well insulated with stainless steel, and it had dimensions of 3 m × 4 m × 5 m (height × width × length). The heat sources were independently controlled electric heaters with fins, and the humidity sources were controlled using dry stream humidifiers. The chamber was equipped with real-time monitoring equipment to monitor indoor environmental parameters, and it was controlled in real time through an automatic control system. The controllable range of the ambient air temperature (Ta) was −20 °C~85 °C, with an accuracy of ±0.5 °C, and the controllable range of the relative humidity (RH) was 20~98%, with an accuracy of ±3%. The climate chamber had no external walls, windows, or doors, and there were no temperature deviations between the inner wall and the indoor environment. Hence, the mean radiant temperature was equivalent to the inner air temperature [9], and participants were not affected by radiant heat in the chamber. To provide a site for rest and preparation, we also built a preparation area. To maintain the participants’ thermal comfort, the temperature/humidity in the preparation area was set to approximately 26 °C/50%, as a previous study found these values to be neutral and comfortable for the human body [9]. The layout of the chamber is shown in Figure 1.

2.2. Participants

G*Power software (https://www.psychologie.hhu.de/arbeitsgruppen/allgemeine-psychologie-und-arbeitspsychologie/gpower, accessed on 8 June 2025) was used to calculate the sample size of participants before the experiments were conducted. Referring to statistical requirements and previous studies [27], the following were selected: a required power level 1 − β of 0.8, a significance level α of 0.05, and an effect size of 0.25. After calculations, the smallest sample size was determined to be 24 for the whole experiment. Considering the duration and budget of each experiment, forty-three non-heat-acclimated young male participants were recruited (mean ± SD: age = 24 ± 3 years; weight = 75.1 ± 7.6 kg). Previous research [13,28] has found that this sample size is adequate for detecting a moderate effect size of heat acclimation on humans. The participants mainly include young students and athletes, all of whom have good health conditions and physical fitness. The participants had no musculoskeletal injuries or cardiovascular, metabolic, renal, or respiratory diseases, and they were not tobacco users. All participants had lived locally for a few years and had adapted to the climate. Due to the inherent variations in subjective evaluations, only male participants with more than a year of working experience were selected to ensure as much homogeneity as possible. These participants did not perform any physical tasks or manual labor in the six weeks prior to the experiment. Therefore, they were considered not to be acclimatized, as it is generally considered that 6 weeks is sufficient for de-acclimation [4]. During the first visit, the participants completed baseline testing, which included measurements of height (using ultrasonic height measuring instruments, ±0.005 m, YIPIN EF08B, Guizhou, China) and weight (using an electronic high-precision balance scale, ±0.001 kg, HCE2010, Shanghai, China); VO₂max (using a PowerLab exercise physiology system); and self-reported physical activity. Based on these data, the participants walked at 5 km h⁻¹ with the treadmill incline adjusted to 45% VO₂max during the HST and heat acclimation training. All experiments in this study were approved by the Ethics Committee of Tianjin University of Traditional Chinese Medicine (grant number: TJUTCM-EC20110004), and all participants provided informed consent in accordance with the Declaration of Helsinki.

2.3. Experimental Design and Process

The protocol comprised two heat stress sessions and a 90 min cycle ergometry training session conducted over 10 consecutive days. A 10-day, low-volume training program conducted in a laboratorial context was chosen for heat acclimation training because it is widely believed that 10-day training is adequate for heat acclimation [5]. A pre–post-test experimental study with randomized groups was performed to evaluate the effects of heat acclimation on the human body in different environments. A repeated-measures and within-factors design was used to ensure that the participants became heat-acclimated in the same hot environment and to investigate the effects of heat acclimation training in different hot environments. The whole experiment was conducted from October to December in northern China, where outdoor conditions were 5.4 ± 2.2 °C and 57 ± 5.2%. No participants reported work or frequent exercise in hyperthermal environments for three months prior to each experiment. The participants were required to be free of disease and injury, and any medication that might affect their thermoregulation system, hydration balance, or psychological cognition was also forbidden. The entire experiment consisted of three parts (Figure 2a): pre-heat acclimation (pre-HA) testing, heat acclimation (HA) training, and post-heat acclimation (post-HA) testing. To exclude order effects and prevent the test sequence from affecting the participants, the environmental conditions pre-HA and post-HA were randomly arranged and designed, and the experimental process adopted a single blind approach.

According to previous research, when the ambient temperature is above 32 °C and the ambient relative humidity is greater than 40%, the setting can be considered a high-temperature and high-humidity environment [4]. Therefore, all environmental conditions selected in this study were higher than 32 °C/40% RH. Each participant was required to participate in all experimental testing procedures, including HST and HA training; that is, all participants were required to attend all heat stress tests and heat acclimation training sessions (Figure 2b). During the HST sessions, six ambient conditions were selected: 32 °C/40% (condition I), 32 °C/70% (condition II), 35 °C/40% (condition III), 35 °C/70% (condition IV), 38 °C/40% (condition V), and 38 °C/70% (condition VI). However, the participants exercised in a 38 °C/40% environment during the HA sessions. Each test was identical in terms of preparation and measurement, and each test condition was randomly arranged. The labor intensity was designed to be constant, i.e., during both the HST and HA training, the participants were asked to run on a treadmill at a speed of 5 km/h. The exercise intensity and environmental conditions in the HA intervention were based on the cyclic ergometric HA training method most commonly reported in a previous review [8]. Physiological and psychological indices were recorded during each test. Moreover, all experimental conditions were kept hidden from the participants. All trials were conducted at the same time each day to avoid circadian rhythm effects. Both the test and training sessions lasted for 120 min (including 30 min for preparation). The participants were forbidden from consuming alcohol and caffeine and exercising for 24 h prior to each laboratory visit. They were required to drink an adequate amount of water the night before and the morning of each test to ensure that they were hydrated, and they were required to wear short-sleeved sportswear and sneakers, with a clothing insulation value of 0.5clo [28,29].

Upon arriving at the laboratory, each participant was provided with water, corresponding to 7 mL kg⁻¹ body weight [30]. Before exercising, the participants remained sedentary in a thermoneutral environment (26 °C, 50% RH) for 30 min to stabilize physiological and psychological variables [31]. They then entered the chamber for pre-exercise measures and exercised at 45% VO_2max on motorized treadmills. Post-exercise measures were performed immediately after exercise termination. After completing the pre-HA tests, all participants took part in heat acclimation training over 10 days (45% VO_2max, 90 min; 38 °C/40%), followed by post-HA tests similar to the pre-HA tests. After completing each test, the participants left the chamber and dried their bodies with a towel, and drinking water intake and defecation were recorded to calculate whole-body perspiration. The participants rested in the preparation room and then rehydrated before leaving the laboratory.

2.4. Measurements

2.4.1. Environmental Index

To monitor the deviation of environmental parameters, the measurement points and equipment were arranged according to each participant (Figure 1). The device was set to heights of 0.1 m, 0.6 m, 1.1 m, and 1.7 m, corresponding to the heights of the ankle, waist, chest, and head, respectively [30]. The instruments and their accuracy are provided in

Table 1. Details of experimental instruments.

Parameters	Instruments	Range	Accuracy
Air temperature and relative humidity	High-precision temperature and humidity recorder (MX1102A, HOBO, Onset, Beijing, China)	0−50 °C, 1–90%,	±0.2 °C, ±2%
Air velocity	Hot-wire anemometer (Testo, 425, Titisee-Neustadt, Germany)	0–50 m/s	±0.03 m/s
Black-bulb temperature	WBGT-2006 (Huiyi, Beijing, China)	−10–100 °C	±0.5 °C

.

2.4.2. Physiological Index

Heart rate (HR), rectal temperature (T_r), and local skin temperature (T_sk) were measured during each exercise session. Physiological indices were recorded in 15 min intervals. HR is important for assessing the human cardiovascular system, and it was measured using chest-mounted telemetry units (H9, Polar, Kempele, Finland) in this study. As HR would decrease after the participants stopped exercising and is affected by posture, it was measured in a standing position during each test. T_r and T_sk were measured using copper–constantan thermocouples (Omega, Inc., Norwalk, CT, USA). As rectal temperatures remain stable at a depth of 7–16 cm from the anus [32], a thermocouple was inserted 10–12 cm into the rectum when measuring Tr, and the depth of each measurement was the same. Changes in HR (ΔHR) and Tr (ΔT_r) were also obtained as the difference values between the peak and baseline values.

The thermocouples used to measure local skin temperature were taped to the neck, right scapula, left hand, and right tibia (Figure 3), and the overall skin temperature (T_sk) was calculated using Formula (1) [33]:

T_sk = 0.28 × (T_neck + T_scapula + T_tibia) + 0.16 × T_hand,

(1)

Here, T_sk denotes the mean skin temperature; T_neck denotes the skin temperature of the neck; T_scapula denotes the skin temperature of the scapula; T_tibia denotes the skin temperature of the tibia; T_hand denotes the skin temperature of the back of the hand.

Sweat loss was calculated via weight loss. During each test, the participants’ liquid intake and urine excretion were recorded, and they were nude and thoroughly dry before they were weighed. The total sweat loss was calculated as follows: total sweat loss (M_sl) = pre-exercise nude body mass (kg) + fluid ingested (kg) − urine excreted (kg) − post-exercise nude body mass (kg) [34].

2.4.3. Subjective Perception

Subjective perception, including the rating of perceived exertion (RPE) and thermal sensation vote (TSV), was recorded every 15 min. The RPE was measured using the Borg scale [35], and the TSV was recorded using a seven-point scale [30]. To ensure that all participants were familiar with the scales adopted in this study, explanations and training about the content of the questionnaires were provided during the pre-experiment period. In this study, all measured data were averaged over the recorded values, and analyses were conducted using these averaged measurements.

2.5. Statistical Analyses

All data in the present study are presented as means ± SD. The normality of the data was assessed using the Shapiro–Wilk test, and data that violated this assumption were log-transformed prior to ANOVA. Analysis of variance (ANOVA) employing a condition (6 levels: I, II, III, IV, V, and VI) and acclimation (2 levels: pre-HA and post-HA) and a paired t-test were performed to analyze the effects of HA on physiological measures. The Wilcoxon non-parametric test was performed to evaluate the significant differences between the effects on subjective perception. The peak physiological parameters and their changes (difference before and after exercise, Δ) were calculated and studied. If a significant effect of condition or an interaction between acclimation and condition was observed, post hoc comparisons were performed using Tukey’s HSD test. The effect size was calculated and is expressed using Cohen’s d. The small, medium, and large effect sizes were 0.2, 0.5, and 0.8, respectively. Statistical Package for Social Sciences (SPSS) version 24.0 (IBM, New York, NY, USA) was used for all statistical analyses. The significance level was set to p < 0.05.

3. Results

3.1. Environmental Parameters

To eliminate deviations in environmental conditions, the actual environmental parameters in the chamber were monitored. The designed and actual environmental parameters of each condition are shown in

Table 2. Actual environmental parameters of each condition.

Conditions	Designed		Actual
	Temperature/°C	Relative Humidity/%	Temperature/°C	Relative Humidity/%	Air Velocity	ET*
I	32	40	31.9 ± 0.1	39.7 ± 1.8	<0.1 m/s	25.82
II		70	32.2 ± 0.1	70.3 ± 1.9		28.54
III	35	40	35.1 ± 0.1	40.2 ± 2.2		27.96
IV		70	34.8 ± 0.2	70.4 ± 2.1		31.17
V	38	40	38.2 ± 0.1	40.3 ± 1.3		30.20
VI		70	38.1 ± 0.2	69.8 ± 2.1		33.97
HA training	38	40	37.9 ± 0.1	39.8 ± 2.3		30.20

. The variables prior to each trial were not different (p > 0.05 for all), which suggests that the physiological status of all participants was similar at the beginning of all tests. Moreover, the ambient temperature (Ta) and relative humidity (RH) did not differ throughout the experiments (p > 0.05). New effective temperatures (ET*) were calculated for the six conditions [36], and the results are as follows: 25.82 °C (condition I), 28.53 °C (condition II), 27.96 °C (condition III), 31.17 °C (condition IV), 30.20 °C (condition V), and 33.98 °C (condition VI). Clearly, the actual air temperature and relative humidity were very close to the designed environmental conditions. Thus, the indoor thermal environments met the requirements.

3.2. Physiological Parameters

3.2.1. Rectal Temperature

The peak rectal temperatures (absolute data) are presented in Figure 4. ANOVA revealed that there was a significant main effect of acclimation (p = 0.032, η²p = 0.722) on the peak rectal temperature (T_r, max). A comparison revealed that T_r, max decreased by about 0.3 °C (95%CI: 0.04 °C–0.55 °C). The results suggest that the effects of environmental conditions and the interaction effects between acclimation and condition were not significant (p = 0.97). This indicates that the decreases in Tr, max after acclimation were similar under different environmental conditions. In terms of Figure 4, this means that the improvement in T_r, max due to acclimation was not significantly affected by environmental conditions.

Increases in rectal temperature (ΔT_r) were also calculated post-HA (at the end of exercise minus that at the start). An acclimation effect (p < 0.001) and a condition effect (p = 0.003) were observed for ΔT_r but without interactions (p = 0.419) between acclimation and condition (Figure 5a). Under conditions I, II, III, and V, ΔT_r was significantly lower post-HA than pre-HA (I: 0.94 ± 0.38 vs. 1.53 ± 0.34, p = 0.027; II: 1.14 ± 0.39 vs. 1.76 ± 0.48, p = 0.035; III: 1.16 ± 0.39 vs. 1.64 ± 0.48, p = 0.11; V: 1.08 ± 0.24 vs. 1.54 ± 0.38, p = 0.03). No significant difference was observed between conditions IV and VI, although ΔT_r was lower than pre-HA (IV: 1.02 ± 0.45 vs. 1.80 ± 0.57, p = 0.16; VI: 1.54 ± 0.48 vs. 2.60 ± 0.45, p = 0.12). This indicates that acclimation had different effects on ΔT_r depending on the conditions.

3.2.2. Heart Rate

There was a significant main effect of acclimation on peak heart rate (p = 0.002, η²p = 0.926), indicating that acclimation can mitigate cardiovascular stress in hot environments (Figure 6). A comparison revealed that, after acclimation, HRs decreased by approximately 8.2 bpm (95% CI: 5.0 bpm–11.4 bpm). The interaction between acclimation and condition was significant (p = 0.037, η²p = 0.652), indicating that acclimation had different effects on the peak HR depending on the environmental conditions. The decrease in heart rate had a wide range, from 2.4 to 13.4 bpm.

An acclimation effect (p < 0.001, η²p = 0.943) and a condition effect (p < 0.001, η²p = 0.750) were observed for ΔHR but without an interaction between acclimation and condition (p = 0.818, η²p = 0.067) (Figure 5b). A paired t-test was also performed between pre- and post-HA. ΔHR was significantly lower post-HA than pre-HA under all environmental conditions (A: 32.4 ± 8.35 vs. 45.2 ±10.50, p = 0.031; B: 29.0 ±10.70 vs. 35.4 ± 9.94, p = 0.039; C: 26.8 ± 9.93 vs. 38.8 ± 8.61, p = 0.015; D: 33.2 ± 5.97 vs. 45.4 ± 14.4, p = 0.034; E: 31.6 ± 6.01 vs. 42.4 ± 10.12, p = 0.01; F: 45. 4 ± 11.59 vs. 66.0 ± 8.97, p = 0.01). This indicates that the acclimation effects on ΔHR were similar and significant under different environmental conditions.

3.2.3. Skin Temperature

The skin temperature (T_sk) of all participants was also analyzed (Figure 5c). The skin temperature varied under different environmental conditions. An acclimation effect (p < 0.001, η²p = 0.612) and a condition effect (p < 0.001, η²p = 0.530) were observed for ΔHR but without interactions between acclimation and condition (p = 0.625, η²p = 0.077). T_sk was significantly lower post-HA than pre-HA under all environmental conditions (I: 32.83 ± 0.38 °C vs. 33.68 ± 0.48 °C, p = 0.01; II: 33.09 ± 0.18 °C vs. 33.92 ± 0.40 °C, p = 0.01; III: 30.84 ± 0.39 °C vs. 34.41 ± 0.21 °C, p < 0.001; IV: 34.32 ± 0.22 °C vs. 35.01 ± 0.27 °C, p < 0.001; V: 34.36 ± 0.29 °C vs. 34.87 ± 0.24 °C, p < 0.001; VI: 35.81 ± 0.67 °C vs. 36.23 ± 0.47 °C, p = 0.04). This indicates that the acclimation effects on ΔHR were similar and significant under different conditions.

3.2.4. Sweat Loss

Sweating has a considerable impact on heat dissipation in hot environments. As shown in Figure 7, the sweat loss before and after acclimation differed, suggesting that the acclimation effect was not independent. Before acclimation, the sweat loss at 40%RH was higher than that at 70% RH under all environmental conditions. However, after acclimation, the sweat loss under the 38 °C−70% RH conditions showed the opposite phenomenon. This indicates that the acclimation effect on sweat loss differed under various environmental conditions.

Sweat loss (M_sl, Figure 5d) was found to be significantly higher post-HA than pre-HA under conditions IV and VI (IV: 0.85 ± 0.058 kg vs. 0.79 ± 0.076 kg, p = 0.034; VI: 1.06 ± 0.064 kg vs. 0.81 ± 0.063 kg, p = 0.001), while acclimation was effective in reducing M_sl post-HA (I: 0.75 ± 0.02 kg vs. 0.78 ± 0.32 kg, p = 0.215; II: 0.69 ± 0.032 kg vs. 0.72 ± 0.038 kg, p = 0.039; III: 0.88 ± 0.024 kg vs. 0.99 ± 0.017 kg, p < 0.001; V: 1.01 ± 0.05 kg vs. 1.05 ± 0.06 kg, p = 0.03).

3.3. Psychological Parameters

The final RPE responses under all environmental conditions are presented in Figure 8, and they are inconsistent with physiological responses, as illustrated in Figure 8. Although both final RPEs were slightly lower post-HA than pre-HA, acclimation did not cause a significantly lower final subjective perception for all environmental conditions (p > 0.05). Although the TSV values post-HA were lower than those pre-HA under all environmental conditions by the end of the HST (Figure 4b), no differences in the TSV values at the end were observed between the post-HA and pre-HA trials (p > 0.05 for all).

4. Discussion

The aim of this study was to determine the impact of training environment on physiological markers related to heat stress during short-term heat acclimation (STHA), as well as exercise ability in high temperatures after 90 min of heat stress training. To the best of our knowledge, the majority of the heat acclimation literature is based on the same or similar training and testing environmental conditions; thus, this is the first study to evaluate the effects of HA training under diverse conditions. The main finding of this study is that heat acclimation significantly improved HR and T_sk under six environmental conditions designed to reflect dynamic environments. Moreover, T_r also significantly improved under environmental conditions I, II, III, and V, while it did not significantly improve under conditions IV and VI. Moreover, after HA induction, unlike T_r, HR, and T_sk, M_sl decreased under conditions I, II, III, and V but increased under conditions IV and VI. Although no statistical difference was observed, HA treatments resulted in reduced final RPE and TSV responses under all conditions pre- and post-HA. Therefore, it is beneficial to exercise for several days in high-temperature environments [37], and to not underestimate the importance of HA strategies, we also propose that targeted training may be an effective method of HA.

As we conducted our studies in a laboratory, the results need to be interpreted while considering several experimental characteristics: (1) The intensity of the HST and HA training was well controlled, but it was impossible to stringently control the health status of all participants. However, the intensity used in this study was convenient, easy, and acceptable for our participants due to it being the same as routine training methods in daily life. (2) All experiments were carried out in an environmental chamber, and the environmental conditions were well controlled. However, according to previous research [38], actual environmental conditions are much more complex; therefore, this should be considered in practical application to ensure the effective application of HA training. (3) Based on previous research [4], as the most common and direct subjective perception parameters, only thermal sensation and the rating of perceived exertion were assessed in the present study; however, this may limit the interpretation of the data.

The observations of the present study are consistent with those of previous studies that reported significant benefits of heat acclimation [39]. Heat acclimation is a helpful measure for reducing risk factors in hot environments. Relevant research [16] has demonstrated that heat acclimatization significantly improves the adaptability of the human body to extremely hot environments. Shen et al. [1] conducted an HA training study, and the results indicated that T_r and HR decreased by 1.3% and 17.7% after HA training in a hyper-thermal environment. Then, they used the Cox regression method to analyze heat acclimatization effectively and found that it was significantly affected by both temperature and relative humidity. A study conducted by Wagner et al. [40] evaluated heat tolerance and the acclimatization of high-temperature work, but the heat acclimation protocol was not restricted. Thus, reductions in rectal temperature could not be attributed to the different heat acclimation protocols. Luke Pryor et al. [28] studied the effect of intermittent exercise–heat exposure after initial HA. The results showed that, after a month, HA continued to affect HR and T_r, and it reduced perceptual and physical strain during exercise. However, most previous studies have only focused on the impact of heat acclimation on physiological parameters, with few examining its effects on subjective perception, especially the behavioral performance of the human body under different environmental conditions. To sum up, heat acclimation has significant benefits that can improve the risk factors of the human body in hot–humid environments; however, according to the results of this study, the selection of more advantageous HA training conditions has better effects on reducing the physical load and physiological strain of personnel.

There is little research on the effects of HA on human body parameters during continuous exercise in hot environments, although higher heat stress could accelerate T_r adaptation [9]. It should be pointed out that heat acclimation in higher-temperature and -humidity environments can lead to better adaptation to activities under relatively lower environmental conditions. This indicates that environmental heat stress affects not only the magnitude of heat acclimation but also its effectiveness. Tian [16] also obtained a similar finding, observing that the decrease in T_rafter HA is dependent on the environmental temperature. In addition, another study [41] found that the decrease in peak rectal temperature during HA training is related to the dry-bulb temperature and relative humidity. Given the results of this study, both an increase in a single environmental factor and an increase in total thermal stress may affect the magnitude of HA.

One of the most noteworthy observations of the present study is that acclimation significantly lowered the exercise T_r value under certain environmental conditions, i.e., the acclimation intervention was more effective in mitigating the heat strain of the human body in environments with a lower ET* (I, II, III, and V). In these environments, ΔT_r significantly decreased, which, in turn, could lower cardiovascular strain and the risk of heat-related diseases caused by excessive body heat storage. Repeated exposure to high temperatures can induce changes in physiological function caused by endocrine factors activated by exercise [42]. Endurance-trained individuals behave physiologically as if they are already heat-acclimatized and in a position to adapt to lower-ET* environments. Previous acclimation studies conducted pre- and post-tests in the same environment [1,16]; however, this induction is likely to have different effects in the different environments tested in the present study. Another noteworthy observation is that acclimation did not appear to reduce M_sl under all conditions. Instead, a lower M_sl was only observed in lower-ET* environments. In contrast, when exercising in higher-ET* environments, M_sl increased. Heat acclimatization has benefits in improving the sweat production ability to adjust body temperature in higher-ET* environments. However, this means that rehydration during training needs to be taken seriously [33].

ΔHR post-HA significantly differed from that pre-HA. Improvements in HR were observed under all conditions. The heart rate adaptation observed under condition E is also applicable to the other conditions in the current study, but for higher-ET* environments, the results are still unknown. Cardiovascular strain is a limiting factor for work tolerance when working in hot environments [43]. The different T_sk responses could be due to underlying HR differences between pre-HA and post-HA under different conditions during exercise–heat stress. HA training helps improve plasma volume (PV) [8] and cardiac stroke volume (CSV) [36]. By increasing skin blood flow, convective and evaporative heat transfer across the skin surface can be improved. The increased PV and CSV obtained through training contribute to reducing T_sk. In this present study, under conditions I, II, III, and V, the lower ET* further contributed to heat loss, which was enough to remove excess heat without increasing sweating.

Although the RPE and TSV were not found to be significantly reduced under each condition, the final RPE and TSV post-HA were still lower than those pre-HA, as mentioned above. This is consistent with a previous study [43]. These improvements in perception may still help improve human performance in hot environments [44]. A reduced human strain response may lead to an improvement in thermal comfort, which will enhance the individual’s motivation and ability to continue working.

When discussing human responses, six main factors are evaluated: the metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, and humidity. Additionally, there are several other independent factors, such as the individual’s emotions and thermal history. Human health and safety with regard to other environmental variables, for instance, air velocity and thermal radiation, may also be affected by short-term HA [9]. Further in-depth research should be conducted to understand and fill these gaps in the future. The purpose of this study was to compare the effects of a 10-day active heat acclimation training strategy in ensuring employees’ safety performance rather than to examine the uniqueness of heat acclimation. Therefore, future research should attempt to investigate whether intermittent exposure can be used to maintain heat/thermal adaptation, in addition to the impacts of environmental conditions on adaptability, active training forms, and training intensity. Future research should investigate whether changes in protocols (e.g., training time, frequency, methods, etc.) affect the integrity of acclimation, retention time, or sensitivity to acclimation attenuation.

In this study, heat acclimation appeared to be more effective in reducing heat strain during low–moderate-intensity work. Unreasonable HA training may result in a restrictive state, which may further increase risks in hot environments. Therefore, personnel in high-temperature environments must also consider the environmental conditions during training. Higher-ET* training conditions may be more effective and could reduce the adverse effects of heat stress on human health and safety. This study complements recent studies aiming to expand the understanding of the effects of heat acclimation in occupational contexts [8,28].

This study has some limitations. A major shortcoming of this study is the small sample size. Further research should include more sample sources, including people of different ages, genders, and regions. In addition, due to limitations in the experimental environments, the subjects in this study were only young males or athletes; thus, the results of this experiment are temporarily applicable only to the young male population. In future research, more participants will be recruited, including those from different industries, genders, BMI, and regions. Moreover, more environmental conditions and forms of exercise need to be considered in future research. In this study, the TSV was evaluated without considering thermal comfort. However, thermal comfort usually improves due to heat acclimation [1], so it should be considered in future research. Laboratory studies alone are not enough to delve into this topic, and field investigations should be conducted to understand the characteristics and practicability of HA in actual industrial buildings. Lastly, due to time and lab conditions, many physiological measurements could not be measured and need to be further investigated. Additionally, the relationship between ET* and thermoregulatory responses is very important and worthy of in-depth research.

5. Conclusions

This study revealed the effects of heat acclimation on human responses by conducting studies in humans. HA can effectively improve human physiological parameters in high-temperature environments. The results indicate that reasonable HA training can significantly reduce risk factors and ensure human safety, and a heat acclimation training program designed for higher-ET* environments can be effectively applied in lower-ET* environments. The adaptation rate of T_r decreased by about 0.3 °C, and HR decreased by approximately 8.2 bpm. Moreover, T_sk also significantly improved after HA training. In hot–humid environments, heat acclimation can reduce thermal sensation and the rating of perceived exertion during exercise, but it is not applicable to the final RPE and TSV. Critical conditions should be considered in the process of HA training. When conducting HA training in an environment with a relatively high ET, better improvement results can be achieved. However, the upper limit of this training environment still needs further research. These results lay a theoretical foundation for the establishment of a reasonable training program. These beneficial changes are very important to protect people in hot environments.

In our study, the participants underwent a short-term heat acclimatization training program. Such a scheme can be easily implemented in advance, and it does not affect actual production. Our study provides clear empirical evidence supporting the future development of reasonable short-term HA training, either alone or in combination with other techniques. This type of HA training has obvious practicability, and it is of great significance in preventing excessive fatigue and reducing heat risk.